Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L49/00—Packet switching elements
  • H04L49/50—Overload detection or protection within a single switching element
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L49/00—Packet switching elements
  • H04L49/30—Peripheral units, e.g. input or output ports
  • H04L49/3036—Shared queuing
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04Q—SELECTING
  • H04Q11/00—Selecting arrangements for multiplex systems
  • H04Q11/04—Selecting arrangements for multiplex systems for time-division multiplexing
  • H04Q11/0428—Integrated services digital network, i.e. systems for transmission of different types of digitised signals, e.g. speech, data, telecentral, television signals
  • H04Q11/0478—Provisions for broadband connections
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L12/00—Data switching networks
  • H04L12/54—Store-and-forward switching systems 
  • H04L12/56—Packet switching systems
  • H04L12/5601—Transfer mode dependent, e.g. ATM
  • H04L2012/5638—Services, e.g. multimedia, GOS, QOS
  • H04L2012/5646—Cell characteristics, e.g. loss, delay, jitter, sequence integrity
  • H04L2012/565—Sequence integrity
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L49/00—Packet switching elements
  • H04L49/20—Support for services
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L49/00—Packet switching elements
  • H04L49/30—Peripheral units, e.g. input or output ports

Definitions

• This invention relates to a method and a unit for the reconstruction or resequencing of the initial order of the sequence of a cell flow in telecommunication systems using a special numerical technique called ATM (Asynchronous Transfer Mode), for the transmission and/or switching of the vocal signals, video signals and data.
• ATM Asynchronous Transfer Mode
• the information relating to the various users and services are organized as time contiguous units with a fixed length of approx. 400 bits called cells.
• the cell bits are subdivided into a field containing the information to be exchanged (payload) and into a heading field (header) containing the information requested to route the cell through the switching network and other service information.
• the ATM technique is considered as the more profitable solution for the multiplexing and switching into the evolution of the public communication networks towards the future wide band numerical network integrated in the services (B-ISDN), as this technique allows a very great flexibility compared to the previous systems.
• B-ISDN wide band numerical network integrated in the services
• a virtual channel is a logic association between the end points of a link that enables the unidirectional transfer of cells over that link
• a virtual path is a logic association of virtual channels between the end points of a link that enables the unidirectional transfer of cells belonging to a set of virtual channels to take place over that link (CCIT Rec. 1.113).
• the integrity of the cell sequence inside every VC/VP has to be preserved, i. e. the ATM network must be able to supply at the output the cells belonging to th same VC/VP in the same sequence order with which they ar sent to the network.
• the keeping of the sequencing order is automaticall assured if all the cells in the same VC/VP follow the sam path inside the ATM network.
• a switching nod consists (Fig. 1) of a switching " network (SN) and of certain number of exchange terminations (ET) to which th
• This operation hereinafter referred to as "resequencing" is conceptually simple in that it consists in receiving an inlet disordered cell sequence and, on the base of an information contained in the header of the cell in the proprietary format internal to the node, to determine, for each VC/VP, the correct position of the cell in the output sequence so to supply the cells of each VC/VP in the same order they had entered the network.
• the practical performing of the resequencing operation at ATM level is mainly complicated by the speeds involved and by the number of the virtual channels which may be activated at the same time on the same ATM link. In fact, this number may be very high (even two thousand and more on an ATM link at about 150 Mbit/s) and moreover each VC may need at least 24 bits to be completely identified (to this purpose, see the CCITT 1.361 Recommendation). Therefore, the implementation of the resequencing operation based on VCI/VPI (VC/VP identifier), in addition to be complex in itself, requires even the utilization of a considerable memory size.
• VCI/VPI VC/VP identifier
• the sequencing order at the inlet of the network is then kept at the output if all the cells pass the same period of time inside the network, before entering the output link.
• a skip before sending the cell to the output link, it is necessary to wait that, from the moment in which the cell enters the network, a period of time equal to the maximum delay introduced by that has elapsed.
• the value of 10-10 quantile of the maximum delay (DM) introduced by SN as a value of the time which must pass from the moment in which the cell enters the SN, before it is sent on the output link, may be used.
• the resequencing operation shall then be based on the comparison between DM and the time actually elapsed by the cell inside the network, without that it was necessary to take into consideration the pertinent VC/VP.
• the cell is then let to wait for the remaining time, given by the above mentioned comparison, so to emulate its exit from SN network after a period of time not less than DM.
• the time elapsed by the cell inside the SN network is obtained from the time stamp (TS) which is stored in the internal header of the cell when it is in the inlet ET, before its sendin to SN network.
• TS time stamp
• first list consisting of the values representing the cell memory locations, such list bein addressed by three pointers, respectively a first pointer identifying the input time slot in which the cell is received, a second pointer identifying the output time slot for which the output of the received cell is foreseen and a third pointer, or virtual pointer, identifying the output time slot in which more than one cell should be supplied;
• the invention consists of a unit for the reconstruction of the initial order of the flow sequence of the cells pertaining to the same virtual channel, entering the switching network of an ATM telecommunication system, such unit being positioned between the output of the switching network and a heading processor and including:
• the intermediate storage includes a cell memory in the locations of which the output cells from the switching network are stored and by the fact that the controller includes: - a first storage containing the pointers at the locations of the said cell memory, - a second storage in which some queues are formed, with elements each of those addresses the cell in the previous position of the same queue;
• FIG. 2 schematically shows the structure of an ETi exchange termination with a resequencing unit of the initial order of the cell sequence for each VC according to the invention
• Figure 3 shows the more detailed block diagram of the resequencing unit according to the invention
• Figure 4 shows a data structure used in an embodiment of the invention to perform the resequencing operation.
• a shared queues approach is used, so to obtain a significant saving in the dimensions of the storage which is sized on the base of the maximum number of cells actually present in the resequencing unit.
• an ATM switching node includes a switching network (SN) to which several exchange terminations ETI, ET2,.... ETn are connected, that receive the flows of the input cells IN/F and emitting flows of output cells OUT/F, correctly routed in the SN network.
• SN switching network
• VC virtual channels
• V virtual paths
• An ET structure, downstream side (i.e. after the SN network) is shown more in detail in Fig. 2. It includes a CHP (Cell Header Processor) block which receives the output cell flow from SN, processes the cells at ATM level and performs the translation of the cells from the proprietary format internal to the node to an ATM standard format, before these are passed to the physical level (PL) for the transmission on the output link.
• CHP Cell Header Processor
• the resequencing operation is performed by a resequencing unit (RU) which is also present in the exchange termination, which has the specific duty to analyze a TS (Time Stamp) of the internal header of the cell and to make the cell wait for the time necessary to avoid skips among cells of a same VC, unless a sufficiently low probability exists, for instance equal to 10-10.
• RU resequencing unit
• the block diagram of the resequencing unit RU is shown in Fig. 3.
• the behaviour of the unit in question may be summarized as follows: it receives a cell flow from the input channel IC, by means of an input block IN, and stores the data in a RESBUF buffer according to the indications of the RESCTRL controller.
• the RU unit always according to the indications of the RESCTRL controller, draws from the RESBUF the cell to be transmitted, if any, so to supply a cell flow to the output channel OC, to which the RU unit is connected by means of the output block OUT.
• the RU is able to carry on a dialogue with the microprocessor present on the ET for the necessary co-ordination.
• the cells received and transmitted by the RU have a fixed length in the proprietary format internal to the node (for instance, 64 bytes) and the input IC and output OC channels are considered subdivided into temporal intervals having a width equal to the cell time Tc at the channel speed, indicated with IS and OS, respectively.
• the resequencing unit RU performs the following operations: - it receives the k-th cell occupying the i-th tim slot of the input channels IC, indicated with IS(i);
• TD(k) the time (in units of cell times which has elapsed from the moment in which the cell ha entered the network, indicated with TD(k), according to th information contained in the internal header of the k-t cell (TS);
• Some contention conditions of the OC may occur, in that i may happen that more than one cell has to enter the OC i the same temporal interval OS.
• the RU controls th contention conditions by putting the contending cells into queues.
• the OS(j) is free, that is, it has not been assigned to any cell; then, RU creates in RESBUF the queue associated to the OS(j), thus containing first only the cell just received;
• the OS(j) has already been assigned to at least one cell, in general to m cells, for which a contention condition of the OS(j) exists: the RU places the k-th cell into the second one, or more generally, into the (m+l)-th position of the queue associated to the OS( ) ; with m the number of cells already present in the queue, the transmission of which has thus already been scheduled in the OS(j), has been indicated.
• the RU enters the queue t be supplied during the transmission:
• some cells may have a null dela jitters.
• the RU manages a priorit information (for instance, a bit or a word in a give field, associated with the TS), contained in the interna header and according to this information it organizes the cells into a high priority queue.
• a priorit information for instance, a bit or a word in a give field, associated with the TS
• Such queue provides a position in correspondence with each OS, in which a high priority cell (and only one) may be stored.
• the RU assigns the position in the queue as previously indicated; if, on the contrary, the cell requires a null delay jitters, the RU determines the OS value it has to engage and schedules its transmission by placing it in the high priority queue corresponding to that OS.
• the RU verifies if a cell is present in the high priority queue, in correspondence with the actual OS, in which case it transmits the cell; if no cell exists in that position, the RU manages the queues of the cells which admit a not-null delay jitters, as previously indicated.
• the dimensioning of the whole data structure essentially depends on the difference between 10- 10-quantile of the maximum delay (DM) and the SN network crossing minimum delay (Dm) .
• MJD Maximum Jitters Delay
• MJD (DM - Dm) (1) expressed in number of cell times Tc or time slots.
• the transmission speed of the input channels IC and the output channels OC are the same, therefore, a time slot can accept only one input cell and one only output cell can be transmitted from the RU if a cell exists that has passed a period of time equal to DM from the moment of its entering the network.
• a cell is transmitted on the OC at the latest after (DM - Dm) time slots from its entering the resequencing unit RU, but however, after (DM - Dm) time slots a position makes itself free inside the RU. If all the cells suffer a Dm delay in the SN network (empty network) , in the RU the maximum number of accumulate cells is equal to : (DM - Dm) (cells) (2) apart from the number of queues present in the. RU.
• Fig. 4 now a preferred executive form of the invention is shown. According to the invention, the pre-setting or scheduling of the transmission of the output cells from RU is obtained by managing the slot IS and OS by means of a list addressed by pointers.
• a first pointer called PZOS (Pointer Zero Output Slot) is used for the management of the global time, therefore it is increased at each cell time Tc.
• PZOS Pointer Zero Output Slot
• This pointer identifies the input slot IS in which the cell is received as well as the output slot OS, contemporaneous to it, so it corresponds to the index appearing in the previous relations.
• This pointer identifies inter alia the output slot OS for which is scheduled the transmission of the cells that no longer need to wait (a part from the contentions) before being transmitted on the output channel, because they have already passed DM cell times in the network (so having Z equal to zero) as well as because they have exhausted their remaining waiting period in the RU.
• PSOS Pointer Scheduled Output Slot
• the input slot IS and the output slot OS may only be addressed in a temporal window subdivided into slots having a Tc width, that has to be wide enough to allow the scheduling of the transmission of the cells placed at (DM - Dm) time slot from PZOS, without the occurring of any skip.
• a queue can contain (DM - Dm) cells: this occurs when all the cells present in the RU have contemporaneously entered the SN network-.
• PZOS and PVOS may differ for (DM - Dm) positions, and therefore, the number of the OS that may be addressed and, consequently, the size of the window has to be equal or greater than:
• TSlotList (Time Slot List), appearing in Fig. 4, addressed by the PZOS, PSOS and PVOS pointers, which represents the part of the temporal axis to which the slots may be addressed.
• Each record of this list that is, each location of the storage the list consists of, is then assimilable at a time slot, so that the number of the records of the TSlotList is defined by Eq.3.
• the cells are stored in the RU until the time they passed in the SN network plus the one they passed in the RU is equal to DM and to solve the contention conditions on the OSs, the contending cells are organized in queues; so that they have, at most, some cells stored in:
• each OS output slot a queue should be associated, sized on the maximum number of cells it can contain.
• the number of the OS slots is equal to 2 • (DM - Dm) + 1 and each queue should contain up to (DM - Dm) cells, so that to store the cells in the RU by means of this kind of approach, an area should be reserved, equal to : 2 • (DM -Dm)2 + (DM -Dm) (cells) (4)
• the number of the queues contemporaneously active is equal to the number of OSs for which the transmission of at least one cell has been scheduled, so that, at most, you may have a maximum of (DM - Dm) queues (see relation 2).
• DM - Dm maximum of (DM - Dm) queues
• the RU stores the cells into a CellMem (Cell Memory) associated to the RESBUF block of Fig. 3, which is shared among the various queues, the dimensions of which is then equal to:
• each queue are organized in stacks; in fact, the queue are supplied with LIFO approach (Last In First Out) i order to facilitate their management, as in this way it i sufficient to know the position of the last .cell of th queue and the link to the cell engaging the preceden position, while a FIFO (First In First Out) managemen needs some information relating to the first as well as t the last cell of the queue.
• LIFO approach Last In First Out
• each TSlotList .record a pointe called PLC (Pointer Last Cell) which addresses the las cell of the queue is stored.
• PLC Pointer Last Cell
• An empty queue from a CAP value associated with PLC acting as terminator an indicating the end of the queue is identified.
• the queues of the lists are separated by empt elements consisting of terminators in the form o univocally recognizable codes.
• LListMem Linked List Memory
• This memory is used also to store the list of the fre cells locations (Free Cell List) , the first of which i pointed by FlpHead (Free cell List Pointer Head), which is contained in a proper register.
• PPC Pointer Previous Cell
• the first cell of each queue is identified by a CAPl value associated with a PPC acting as a terminator, as this cell is not preceded by any other cell.
• PNQ Pointer Next Queue
• PNQ a linked list structure, in which th elements of each list consist of the queues "validated” b PZOS, where this term indicates the active queues (that is the ones containing at least one cell) to which the cell that have exhausted their time in the RU pertain, is the implemented.
• the "validated" queues are al the active queues already addressed by PZOS and not yet b PVOS.
• the separating element among the queues has bee indicated with CAP2 as, for addressing reasons, it consist of one bit more than the preceding separators.
• NxtQList Next Queue List
• PNQ pointers the list of the PNQ pointers to the queues to b supplied subsequently to the actual one (that is, pointe by PVOS) is stored
• PLQ pointer Pointer Las Queue
• the NxtQList has the same depth of LListMem, tha is, (DM - Dm + 1) record.
• the cells identified as high priority cells, which the require a null delay jitters, are stored into a separat queue, dedicated to them.
• the PrioList is addressed by two pointers PPZOS (Pointe Priority Zero Output Slot) PPSOS (Pointer Priorit Scheduled Output Slot), having the same meaning, for th high priority cells, of the PZOS and PSOS. pointers, use for the low priority cells, but having a different size i that they must address a deep memory (DM - Dm + 1) instea of 2 • (DM - Dm) + 1.
• the presence of the queue with high priority cells then involves an increase of the complexity of the RESCTRL block, but does not involve an increase of the dimensions of the Cellmem and then of the RESBUF, in that such cells share the same memory area with the low priority cells.
• the RU fixes equal to DM the value of the delay suffered by the cell in the network, a part from the value of TS; in this way, the cell is placed in the queue pointed by PZOS and therefore it may be immediately transmitted at the output, if no cell exists to be transmitted, the transmission of which has been scheduled for preceding OSs and if no high priority cell exists which must engage the same OS.

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• Engineering & Computer Science (AREA)
• Computer Networks & Wireless Communication (AREA)
• Signal Processing (AREA)
• Data Exchanges In Wide-Area Networks (AREA)

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Cited By (2)

Citations (2)

• 1991
  • 1991-11-28 IT ITMI913192A patent/IT1252021B/it active IP Right Grant
• 1992
  • 1992-11-05 EP EP92923269A patent/EP0614585B1/en not_active Expired - Lifetime
  • 1992-11-05 ES ES92923269T patent/ES2101878T3/es not_active Expired - Lifetime
  • 1992-11-05 DE DE69217555T patent/DE69217555T2/de not_active Expired - Fee Related
  • 1992-11-05 WO PCT/EP1992/002565 patent/WO1993011621A1/en active IP Right Grant

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